1. Field of the Invention
The present invention relates to a rear projector, and more specifically to a rear projector that uses, for example, a digital micromirror device or an LCD (Liquid Crystal Display) as a display device and that projects, on an enlarged scale, an image on the display device surface obliquely onto a screen surface with a projection optical system.
2. Description of Related Art
There have been growing demands for slimming-down of a rear projector. This slimming-down can be achieved by using a compact, wider-angle projection optical system and further providing configuration such that light exiting from the projection optical system is made incident obliquely on a screen at a relatively wide angle of incidence. As a projection optical system having a wide angle of incidence as described above, various types using one or a plurality of curved reflection surfaces have been suggested (for example, see Patent Documents 1 and 2). Using a rotationally symmetric aspherical surface or a rotationally asymmetric aspherical surface (so-called free curved surface) as a curved reflection surface permits ultra-wide angle projection which could have never been achieved with conventional coaxial refractive lenses.
[Patent Document 1] JP-A-2002-196413
[Patent Document 2] U.S. Pat. No. 6,805,447B2
In the rear projectors suggested in Patent Documents 1 and 2, for slimming-down thereof, the degree of oblique projection (angle of incidence on the screen) is increased with a nonaxisymmetric projection optical system, and a free curved surface is used in order to improve the projection performance. However, the use of a free curved surface in the nonaxisymmetric projection optical system results in occurrence of large pupil aberration. Typically, the screen is formed with a Fresnel lens and a lenticular lens. This Fresnel lens is arranged so that the pupil of the projection optical system conjugates with the pupil of an observer, or arranged so that a beam from the projection optical system is not vignetted by a black mask arranged near the image surface of the lenticular lens. Moreover, the Fresnel lens is typically designed to be rotationally symmetric for easier machining. Thus, pupil matching deteriorates between the Fresnel lens of a coaxial system and the projection optical system in which aberration such that the pupil position differs between the pupil vertical and horizontal directions occurs. This causes luminance nonuniformity on the image plane, and it is difficult to suppress the luminance nonuniformity with a usual refractive Fresnel lens.